British patent application GB2212753 suggested adding a power transmission train in the tool turret of an NC lathe to drive driven tools of the turret so as to complete turning , drilling, and milling once a workpiece is clamped.
In fact, limited by the diameter of the turret and the workpiece, except for turning tools, very few driven tools can be held on the turret. So only a few and simple drilling and milling operations can be performed after turning.
Second, the tool exchange accuracy of a turret depends on high precision of the turret and the tools. This means higher manufacturing cost, and consuming more time in tool adjustment.
Moreover, all driven tools are located in a center plane through the workpiece axis, so it is impossible to drill holes or mill faces offset from the center plane of the workpiece, like milling a gear using a disc cutter.
German patent application DE3702424A suggested adding a Y axis guide pair and servo driver to make the turret movable in a direction perpendicular to both the Z axis and the X axis, so that the driven tools held on the turret can machine holes or faces offset from the center plane of the workpiece, including a spline shaft or small spur gear.
The disadvantages of this suggestion are: A) due to an additional moving freedom in the Y axis direction, the position accuracy and cutting rigidity between the cutting tool and the workpiece will be lower and the manufacturing cost will higher than with an ordinary turret lathe; B) inability to machine oblique hole, face or mill helical gears or hob gears; and C) limited by turret size, very few driven tools can be employed to perform few and simple drilling or milling.
Japan patent application JP406226510A disclosed a variable angle driven tool attachment which can attach to the turret of a NC lathe, and comprises a body attached to a turret, a driven tool head supporting a driven arbor rotatably mounted on the body, and a transmitting chain transmitting power from a power supply shaft in the turret to the driven arbor. By changing the angle of the driven tool head, the driven tool can drill or mill the workpiece in arbitrary angles in the center plane of the workpiece. This enables an ordinary turning center to machine oblique holes, faces and spherical surfaces.
The disadvantages of this invention are: A) The narrow space in the variable angle driven tool attachment limits the transmitting power, and the size of the driven arbor, so that only small holes or faces can be machined at low efficiency; B) it is not reasonable to attach several such attachments to a turret, so it is difficult to machine an oblique step screw hole; C) not able to machine holes or faces offset from the center plane of the workpiece, not able to mill or hob gears; and D) limited by turret size, very few driven tools can be employed to perform few and simple drilling or milling.
UK patent application GB2215251A published a two work spindle machine tool for machining both ends of a rotationally symmetrical workpiece. This machine tool comprise a first headstock supporting a first work spindle and movable in the Z axis direction, a second headstock supporting a second work spindle and movable in the X axis direction transversely to the Z axis, a drum turret serving the first work spindle indexably supported on a slide movable in the X axis direction; and a star turret serving the second work spindle indexably supported on a slide movable in both the Z axis and the Y axis directions; 5 guide pairs and servo drivers moving and locating the two headstocks and two turrets, 3 index servo drivers to index the two turrets and the second work spindle. In this machine tool, one end of a workpiece clamped on the first work spindle can be turned by turning tools carried on the drum turret, then the workpiece can be clamped automatically by the second work spindle, and the other end of the workpiece is machined by turning tools and driven tools carried on the star turret. By controlling the movement and rotation of the star turret, the driven tools can move to arbitrary positions and angles, so that the oblique holes and faces on workpieces can be machined. When a workpiece clamped on the second work spindle is machined, another workpiece clamped on the first work spindle can be machined at the same time, so that the production of this machine tool is higher.
The disadvantages of this invention are: A) 5-6 precision guide pairs and 8-10 precision servo drivers are employed to realize the two end machining functions, which makes this machine tool cost very high and complicated; B) limited by the turret space, very few driven tools can be employed to perform few and simple drilling or milling.
U.S. Pat. No. 3710466 suggested a NC lathe, the planiform or cylindrical tool magazine of which is arranged a plurality of axial or radial turning tools, the headstock of which holds a rotatable work spindle that can move along the work spindle axis and at least one direction perpendicular to the work spindle axes, by means of the relative movements between the work spindle and the tool magazine in a plane or a cylindrical surface perpendicular to the Z axis, the workpiece can be positioned into the cutting plane of each selected tool sequentially, so as to increase the available tools and complete total turning process of a Complex workpiece.
The disadvantages of this design are: A) the moving freedom of the work spindle in the direction perpendicular to the work spindle axis will decrease the radial rigidity of the work spindle, in which case, the spindle driven system and the unbalance force of the workpiece will cause considerable vibration, and decrease the radial turning accuracy, and limit the spindle power and speed; B) even though driven tools can be arranged on the cylindrical tool magazine indexable in definite angle increments, these driven tools can not be used in arbitrary angles to machine holes, surfaces and gears, and if rotate the cylindrical tool magazine to arbitrary angles, the position accuracy and the rigidity of the turning tools held on the same cylindrical tool magazine will decrease.
In fact, a practical machine component needs a variety of further machining after turning and simple drilling or milling to complete, such as drilling, boring, tapping for oblique holes, milling, hobbing, shaping, planing, shaving, lapping for gears splines, internal gears, spur or spiral bevel gears, hardening, grinding for external or internal cylindrical or tapers or for large or oblique planes or curved surfaces, grinding for spur gears, helical gears, splines, spur or spiral bevel gears, and broaching for internal splines or shaped holes.
According to the above described prior art and conventional technology, a complex practical workpiece can only be partially completed on one turning center or machining center, and then is carried by robots and auto delivering means to subsequent machining centers until it is finished.
It is very difficult and complicated and will cause accuracy loss for different workpieces to relocate and reclamp automatically on the next machining center, and sometimes it is impossible if no special fixture for this workpiece has been prepared. The automatization of this task is still a big, difficult object of present machinery engineering.